1. Field Of The Invention
The present invention relates to a method and apparatus for improving the performance of an hydraulic elevator driven by means of an hydraulic pump controlled by an electric motor, whereby oil is pumped from a container via a main supply duct into a lifting cylinder to move the elevator upwards and the oil is returned in a controlled manner through the pump to the container to move the elevator downwards.
2. Description Of Related Prior Art
The travelling speed of an hydraulic elevator is controlled either in the traditional manner using a control valve (as in U.S. Pat. Nos. 3,842,943 and 4,249,641) or by varying the rotational speed of the electric motor actuating the pump. An example of the latter method is found in British patent publication No. 1,522,044. As the present invention proposes an improvement to the motor-controlled systems, such systems will be considered in the following discussion.
Generally speaking, in a motor-controlled system, the elevator is driven upward in the following manner:
the control system for driving the elevator upward starts operation of the electric motor and the pump and begins to accelerate the motor and pump according to a definite scheme;
the pressure of the oil flowing from the pump opens a check valve placed between the pump and the cylinder, so that the oil flowing into the cylinder begins to push up the piston. As the rotation of the pump is accelerated, the speed of movement of both the piston and of the elevator car increase accordingly;
when the elevator car has reached the desired speed, the electric motor and the pump continue running at a constant speed;
when the elevator approaches the desired floor, the control system reduces the speed of rotation of the motor in such a way as to decelerate the elevator in the desired manner when the car approaches the floor level; and
when the elevator car has reached the level of the desired floor and the electric motor is running so slowly that the pump only produces a flow corresponding to its internal leakages, the spring-loaded check valve between the pump and the cylinder is closed. The motor can now be stopped, whereupon the car, supported by the check valve, remains standing at the floor level.
The downward movement of the elevator is also controlled by means of the electric motor, because this makes it possible to use the same control system as for upward drive:
the check valve, or a lowering valve shunting the check valve, is opened by means of a magnetic valve;
the oil is allowed to flow from the cylinder into the pump, which thus begins to rotate, functioning as an hydraulic motor;
the control system adjusts the speed of the electric motor to brake the pump so as to achieve the desired change in the speed of the elevator car, be it acceleration, driving at constant speed, or deceleration; and
at the final stage of deceleration, when the car is approaching the desired level at a low speed, the control current to the check valve (lowering valve) is cut off, whereupon the valve is closed and the elevator comes to a halt. The supply of electricity to the motor can now be switched off.
It is desirable that the energy transferred from the pump to the electric motor during downward drive should be fed back to the mains. When a squirrel cage motor is used, this only occurs when the motor is running at a speed exceeding its synchronous speed. However, in many cases the oversynchronous lag of the motor together with the internal leakage of the pump result in an excessive speed of downward travel.
These problems can be avoided by using an inverter to control the motor, but this solution is, in most cases, too expensive. For this reason, generative down-drive, with the motor supplying energy to the mains, is usually not employed, but instead the motor is braked in such manner that the mechanical energy received from the pump is converted into heat in the motor. This results in a very low performance of the system and a substantially higher heat stress on the motor than e.g. in a valve-controlled system, in which the motor stands still throughout the down-travel phase.